Tree compositional change in upland hardwood forests aligns with mycorrhizal type

Long-term monitoring in the eastern United States has documented widespread declines in fire-adapted oaks ( Quercus spp.) and concurrent increases in mesophytic species such as red maple ( Acer rubrum ). While these changes are typically attributed to fire exclusion and shifting light and moisture regimes, belowground functional traits—particularly mycorrhizal association—may play a central role in reinforcing mesophication and altering forest resilience. To gain insight into this process, we analyzed nearly five decades of demographic data from upland oak–hickory forests on the Cumberland Plateau in southeastern USA to quantify shifts in species composition, aboveground biomass, size-class structure, and functional traits. We found that fire tolerance varied significantly by mycorrhizal type, with arbuscular mycorrhizal (AM) species more often classified as fire intolerant than ectomycorrhizal (EM) species, whereas shade tolerance was unrelated to mycorrhizal type and was instead higher in fire-intolerant than fire-tolerant species. Over time, AM and ericoid (ErM) species gained importance value (calculated by combining relative density, frequency, and aboveground total biomass) through increases in frequency and stem density, while EM species declined in importance value despite continued biomass accumulation. Fire-tolerant and intermediate shade-tolerant species exhibited similar declines, indicating a regeneration bottleneck in historically dominant EM fire-adapted trees. In contrast, AM-associated species, including broadly tolerant generalists, now dominate small stems and regeneration strata. Although total aboveground biomass increased across all functional groups, regeneration failure among EM species suggests a structural transition masked by demographic inertia of long-lived canopy individuals. These results provide evidence that shifts in mycorrhizal dominance and fire-related traits reinforce mesophication trajectories, constraining oak regeneration and reshaping belowground mycorrhizal dynamics. Integrating mycorrhizal strategies and disturbance-related traits into management frameworks may be critical for sustaining oak ecosystems and restoring fire-adapted forest structure in the eastern United States.